U.S. patent application number 10/942894 was filed with the patent office on 2005-04-21 for esp glass rupture disks, design and manufacture thereof.
This patent application is currently assigned to UNIVERSITA ' DEGLI STUDI DI TRENTO. Invention is credited to Prezzi, Andrea, Sglavo, Vincenzo Maria.
Application Number | 20050084679 10/942894 |
Document ID | / |
Family ID | 34178458 |
Filed Date | 2005-04-21 |
United States Patent
Application |
20050084679 |
Kind Code |
A1 |
Sglavo, Vincenzo Maria ; et
al. |
April 21, 2005 |
ESP glass rupture disks, design and manufacture thereof
Abstract
The present invention concerns the manufacture of ESP glass
rupture disks with failure resistance higher than 320 MPa able to
shatter into microscopic pieces following the stable growth of the
superficial flaws. It also concerns the manufacture of ESP glass
rupture disk having low variability of rupture pressure (p.sub.max)
values, i.e. having pre-determined fixed p.sub.max value. The
present invention is also directed to provide design criteria for
the manufacture of ESP glass rupture disks having a pre-determined
fixed failure resistance and pre-defined rupture pressure
p.sub.max.
Inventors: |
Sglavo, Vincenzo Maria;
(Trento, IT) ; Prezzi, Andrea; (Rovereto (Trento),
IT) |
Correspondence
Address: |
SUGHRUE, MION, ZINN, MACPEAK & SEAS, PLLC
2100 Pennsylvania Avenue, N.W.
Washington
DC
20037-3202
US
|
Assignee: |
UNIVERSITA ' DEGLI STUDI DI
TRENTO
|
Family ID: |
34178458 |
Appl. No.: |
10/942894 |
Filed: |
September 17, 2004 |
Current U.S.
Class: |
428/410 ;
501/70 |
Current CPC
Class: |
F16K 17/16 20130101;
C03C 21/002 20130101; Y10T 428/315 20150115 |
Class at
Publication: |
428/410 ;
501/070 |
International
Class: |
B32B 017/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 19, 2003 |
EP |
03021294.8 |
Claims
What is claimed is:
1. ESP glass rupture disk, characterized in that said rupture disk
has a failure resistance higher than 320 MPa.
2. ESP glass rupture disk according to claim 1, characterized in
that said rupture disk presents a coefficient of variability of the
failure resistance lower than 4%.
3. ESP glass rupture disk according to claims 1, characterized in
that said rupture disk presents a fatigue exponent higher than 40
and a critical velocity lower than 5 .mu.m/s.
4. ESP glass rupture disk according to claim 1, characterized in
that said ESP glass contains Na.sub.2O in weight percent less than
12%, CaO in weight percent greater than 10% and MgO in weight
percent greater than 3%.
5. ESP glass rupture disk according to claim 1, characterized in
that said rupture disk having thickness t, radius R, failure
resistance .sigma..sub.f,biax and being the diameter of said
rupture disk support d, presents a rupture pressure p.sub.max
higher than working operative pressure pop and dependent in an
algorithmic manner by the ratio t/d.
6. ESP glass rupture disk according to claim 5, characterized in
that said rupture pressure p.sub.max depends on said ratio t/d as
an equation of factor h.
7. ESP glass rupture disk according to claim 6, characterized in
that said factor h is equal to 2.
8. ESP glass rupture disk according to claim 5, characterized in
that said rupture pressure p.sub.max depends on said ratio t/d
according to the algorithmic relation: 34 p max = f , biax k 2 ( t
d ) 2 wherein k.sub.2 is a stress factor.
9. ESP glass rupture disk according to claim 8, characterized in
that said stress factor k.sub.2 is equal to 0.3025.
10. ESP glass rupture disk according to claim 5, characterized in
that said rupture disk having the ratio d/t greater than 5 presents
rupture pressure p.sub.max in the range 3 to 30 MPa.
11. Method for manufacturing ESP glass rupture disk, characterized
in that said ESP glass rupture disk is formed by i) annealing a
starting glass, ii) performing a first ion exchange wherein a
plurality of a first plurality of ions comprising a first element
are exchanged with a second plurality of ions comprising a second
element and having a larger radius than said first ions and iii)
performing a second ion exchange wherein a plurality of said ions
are exchanged to a plurality of third ions comprising said first
element, whereby said rupture disk has a failure resistance higher
than 320 MPa.
12. Method according to claim 11, characterized in that said
annealing phase is performed at a temperature in the range of
500-580.degree. C. and for a period of time in the range of 2-24
hours.
13. Method according to claim 11, characterized in that said first
ion exchange phase is performed at a temperature in the range of
400-475.degree. C. and for a period of time in the range of 4-120
hours.
14. Method according to claim 5, characterized in that said second
ion exchange phase is performed at a temperature in the range of
375-425.degree. C. and for a period of time in the range of 0.1-2
hours.
15. Method according to claim 11, characterized in that said first
ion exchange phase exchanges potassium ions for sodium ions and
said second ion exchange phase exchanges sodium ions for potassium
ions.
16. Method for the design of an ESP glass rupture disk subject to
failure at a rupture pressure p.sub.max higher than working
operative pressure p.sub.op, said rupture disk having thickness t,
radius R and failure resistance .sigma..sub.f,biax and being the
diameter of said rupture disk support d, characterized in that said
method includes the operation of evaluating the ratio t/d in an
algorithmic manner starting from said rupture pressure
p.sub.max.
17. Method according to claim 16, characterized in that said ratio
t/d is evaluated as an equation of factor 1/h.
18. Method according to claim 17, characterized in that said factor
1/h is equal to 1/2.
19. Method according to claim 16, characterized in that said
algorithmic relation is 35 p max = f , biax k 2 ( t d ) 2 wherein
k.sub.2 is a stress factor.
20. Method according to claim 19, characterized in that said stress
factor k.sub.2 is equal to 0.3025.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to manufacturing overpressure
safety systems (rupture disks) by means of Engineered Stress
Profile (ESP) glass, starting from commercially available soda-lime
float glass plates, and to the design criteria of ESP glass rupture
disks.
BACKGROUND OF THE INVENTION
[0002] Rupture disks are the sensitive part of pre-set fracture
devices.sup.[1,2]. The rupture disk in the device fails at a given
rupture pressure (or rated fracture pressure), p.sub.max.
[0003] These devices are used e.g. to protect plants and apparatus
against overpressure or underpressure which could lead to
explosions or implosions.
[0004] There are two main applications: pressure containers and
conduits subject to buffering. In the former case the excess
pressure may be generated by unwanted chemical reactions, such as
e.g. high temperature reactions or other factors. In the latter
case, the excess pressure (or underpressure) is produced by
buffering following the brusque closing of a valve (in
hydroelectric plant) or air-lock.
[0005] Conventionally, rupture disks comprise thin metal disks of
various shapes and sizes according to the exact application.
Generally stainless steel, nickel alloy, aluminium-teflon,
nickel-teflon, aluminium and graphite (for high temperature
applications) are used for producing such disks.
[0006] The main fracture devices now in use can be classified
depending on their intended use/characteristics in a number of
types, namely:
[0007] 1. Flanged rupture disks: they have been used for many years
in fire extinguishers, pumps and other pressure systems. Their
rupture pressure range is 0.15 to 2.5 MPa (FIG. 1).
[0008] 2. Standard disks: they are used in hydraulic plant
(pressure conduits and hoses with rapid circulating fluids). With
an auxiliary conduit (to replace the disk), they are quite adapted
for coping with overpressure due to buffering (FIG. 2).
[0009] 3. 3-layer rupture disks: these are used for low rupture
pressures. The value of p.sub.max is the result of the engraved
aluminium cover (FIG. 3).
[0010] 4. High pressure rupture disks: the precise value of the
rupture pressure (40-700 MPa) depends on the diameter and radius of
the pressure cone (FIG. 4).
[0011] 5. Explosion discharge disks: these are adapted for use in
silos with inflammable powder and large tanks with explosive
liquids or gases. The usual rupture pressure range is 0.15-0.30
MPa.
[0012] 6. Inversion rupture disks: unlike the disk types above,
these are convex in relation to the side from which pressure is
exerted. When the rupture pressure is reached, they invert and
break due to a blade fitted at a certain point (FIG. 5).
[0013] In metal rupture disks the actual rupture pressure is
usually .+-.0.25 MPa the rated pressure, with consequent lower
reliability.
[0014] Failure disk devices are generally fitted with alarms (FIG.
6). These supervise safety devices and alarm when the failure disk
breaks. Generally there are two types of devices:
[0015] Closed electrical circuit (standard): this type of device
has a cable loop on the failure disk cutting off the power when the
disk breaks, and triggering the alarm (FIG. 6a).
[0016] Photo electric cell: this type of device has a
led/photodiode below the disk. When the disk breaks the ray of
light is cut off, alarming the system (FIG. 6b).
[0017] To prevent premature alarming, the working pressure
(operating pressure), p.sub.op, must be set considerably below the
rupture pressure, p.sub.max. Generally pop is set to 0.8p.sub.max.
If the stress is variable or the temperature is particularly high
pop is set to 0.6p.sub.max.
[0018] Conventional rupture disks break without splintering, i.e.
without producing shrapnel. This is an advantage since other parts
of the machinery or device are protected against damage. The
evident disadvantage is the small cross-section of the
fluid-carrying hose compared to the diameter of the disk. In many
applications it would be better if the disk shattered, leaving the
hose cross-section unaltered (and equal to the diameter of the
disk). The shattered pieces could be retrieved by suitable devices
which would reduce the hose cross-section much less than in
conventional metal rupture disks.
[0019] U.S. Pat. No. 6,472,068 discloses glass rupture disks for
blocking fluid flow in conduit, pipes or tanks made from
pre-stressed glass with controllable rupture properties having a
failure resistance lower than 270 MPa.
OBJECTS AND SUMMARY OF THE INVENTION
[0020] The object of the present invention is to provide ESP glass
rupture disks with high failure resistance able to shatter into
microscopic pieces following the stable growth of the superficial
flaws and thereby avoiding undesired obstruction of for use in
pressure containers or conduits subject to buffering. Typically,
the glass rupture disks of the present invention present a failure
resistance higher than 320 MPa.
[0021] Another object of the present invention is to provide ESP
glass rupture disk having low variability of rupture pressure
(p.sub.max) values, i.e. having a definite and precise p.sub.max
value.
[0022] Another object of the present invention is to provide design
criteria for the manufacture of ESP glass rupture disks having a
pre-determined fixed failure resistance and pre-defined rupture
pressure.
[0023] The main advantages of the present invention with respect to
the prior art are related to i) the possibility of manufacturing
glass rupture disks that present a pre-defined mechanical
resistance with a narrow dispersion of the failure data (relative
st. dev. of 4%); ii) the possibility of using such disks in
critical conditions due to their damage resistance, fatigue
insensitivity and high fragmentation capability, thus avoiding the
obstruction of the conduit after rupture.
[0024] Moreover ESP glass is a valid replacement for metal
materials used conventionally in the manufacture of rupture disks
in view of the relatively low production costs and insensitivity to
the chemical environment.
[0025] Also, the design criteria are not sensitive to any
limitation on the thickness and diameter of the produced disks.
[0026] According to the present invention, these purposes are
achieved by means of the claims which follow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The present invention will now be described in detail with
reference to the attached drawings, which are provided purely by
way of non-limiting examples and in which:
[0028] FIG. 1. Device with flanged failure disk (from the
Schlesinger catalogue [3]);
[0029] FIG. 2. Standard failure disk device (from the Schlesinger
catalogue [3]);
[0030] FIG. 3. 3-layer failure disk (from the Schlesinger catalogue
[3]);
[0031] FIG. 4. High pressure failure disk (from the Schlesinger
catalogue [3]);
[0032] FIG. 5. An inversion failure disk type safety device (from
the Schlesinger catalogue [3]);
[0033] FIG. 6. Alarms (a) standard type (b) photoelectric cell
type;
[0034] FIG. 7. Mechanical resistance of an annealed and ESP glass
versus the stress rate in distillate water (dynamic fatigue test on
SLS glass bars in 4-point bending configuration according to ASTM
C158-84 norm);
[0035] FIG. 8. Mechanical resistance (rupture test on SLS glass
bars in 4-point bending configuration according to ASTM C158-84
norm) decay versus the indentation load (Vickers) for a ESP
glass;
[0036] FIG. 9. Schematic of the test configuration used for ASTM
F394-78;
[0037] FIG. 10. Schematic for the "Ball on ring" test;
[0038] FIG. 11. A secured plate and k.sub.1 curve depending on
radius;
[0039] FIG. 12. A supported plate and k.sub.2 curve depending on
radius;
[0040] FIG. 13. System to test rupture disks with concentrated load
at the centre. (a) assembly schematic. (b) after assembly;
[0041] FIG. 14. Operating pressure curve for circular glass plates
(re-fired and hardened). Taken from the Glass Engineering
Handbook.sup.[4];
[0042] FIG. 15. Schematic for rupture disk with supported edge and
concentrated load Q, applied at the centre, by means of a ball;
[0043] FIG. 16. Schematic for rupture disk under pressure with
supported edges;
[0044] FIG. 17. Bench tests for rupture disks. (a) assembly
schematic. (b) assembled schematic.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0045] The composition of soda-lime float glass plates employed for
the manufacture of engineered stress profile (ESP) glass and
consequently of ESP glass rupture disks is a fundamental parameter
for the residual stress profile definition. Therefore, the use of
soda-lime float glasses of different compositions makes it possible
to obtain different mechanical performances using the same
thermo-chemical treatment. On the other side, the use of different
thermo-chemical treatments (characterized by different process
times and temperatures) makes it possible to manufacture ESP glass
rupture disks characterized by different values of mechanical
resistance.
[0046] The arrangement described herein being directed to provide
design criteria for the manufacture of an ESP glass rupture disk
with a pre-determinate mechanical resistance exemplifies how acting
on parameters like temperature and time of the thermo-chemical
treatment and on the composition and thickness of the starting
glass is possible to manufacture rupture disks having pre-defined
operative and rupture pressures.
[0047] The composition of the starting soda-lime float glass used
in the present application is reported in Table 1.
[0048] The compounds constituting the soda-lime float glass able to
influence the mechanical characteristics of the ESP glass rupture
disk manufactured therefrom are the amount of Na.sub.2O, CaO and
MgO are reported in Table 1. Specifically, Table 1 shows the
chemical composition (wt %) of the starting glass used in the
arrangement disclosed herein in comparison with the compositions
disclosed in U.S. Pat. No. 6,472,068 and U.S. Pat. No.
6,516,634.
1TABLE 1 starting glass % SiO.sub.2 % K.sub.2O % Na.sub.2O % CaO %
MgO % Al.sub.2O.sub.3 % Fe.sub.2O.sub.3 other Present 71.7 0.8 10.9
11.8 3.2 1.1 0.2 0.3 invention US-A-6 472 068 62 .div. 73 0 .div. 4
12 .div. 15 0.3 .div. 10 0 .div. 3 0 .div. 17 0 .div. 0.2 -- US-A-6
516 634 71 --* 13 10 4 1 --* -- *wt % lower than 1%
[0049] The ESP glass rupture disks produced starting from a
soda-lime float glass having the composition reported in table 1
and according to the process described in U.S. Pat. No. 6,516,634
present a failure resistance greater than 320 MPa and up to 430 MPa
depending on the considered disk thickness. Specifically, it has
been established that for manufacturing rupture disks with failure
resistance greater than 320 MPa, the amount of Na.sub.2O, CaO and
MgO must be lower than 12% in weight and greater than 10% and 3% in
weight, respectively.
[0050] The ESP glass rupture disks manufactured according to the
arrangement disclosed herein tested in a biaxial flexure
configuration, show higher module of rupture (MOR) than
conventional annealed glass and glass undergoing single
ion-exchange treatment. The key feature in this new approach is to
carefully design the residual stress profile in such a way as to
move the maximum compression away from the external surface and to
carefully control the stress gradient in the surface region. As
this approach involved the `Engineering` of the Stress Profile,
these glasses have been named ESP glasses.
[0051] The coefficient of variability (COV) of mechanical strength
for ESP glass rupture disks according to the arrangement disclosed
herein is less than 4%, against 20% for single ion-exchanged glass
or annealed glass rupture disks.
[0052] Such mechanical characteristics are the result of the stable
growth of surface flaws in ESP glass due to the particular apparent
fracture toughness curve related to the residual-stress
profile.
[0053] A direct consequence of the stable growth is the presence of
surface microcracking leading to the shattering of the ESP material
upon rupture. Such behaviour makes the use of the ESP glass
possible in applications requiring a high rupture value, excellent
reliability and fine fragmentation. These features, coupled to
almost total fatigue insensitivity, make it suitable for many
safety applications incorporating rupture disks.
[0054] The diagram of FIG. 7 shows the failure resistance versus
the stress rate for a 4-point bending test conduced in aggressive
environment (dynamic fatigue test). It is evident that the ESP
glass resistance does not change for all the considered stress rate
values. In the case of the annealed glass the mechanical
performance is instead strongly dependent on the stress rate.
[0055] As regards damage resistance, FIG. 8 shows that the failure
resistance of the ESP glass is insensitive to the size of Vickers
indentation defects, at least up to a load of 5 N. This behaviour
has not been observed for annealed glass.sup.[5].
[0056] The design criteria for ESP glass rupture disks proposed
herein allow to manufacture rupture disks with definite rupture
pressure as a function of parameters like thickness, diameter and
mechanical resistance.
[0057] The proposed criteria do not assume any limitations on the
thickness and diameter values providing that the ratio d/t is
greater than 5, according to the condition of the applicability of
equation 32. Conversely, in U.S. Pat. No. 6,472,068 an operative
range of 1.78.div.3.18 mm for the thickness and 12.7.div.38.1 mm
for the diameter are required.
[0058] With a ratio d/t greater than 5, the range of rupture
pressures (at the verification stage) is between 3 MPa and 30 MPa
(30-300 atm).
[0059] Mechanical Features of ESP Glass Disks
[0060] Production of ESP Glass Disks
[0061] The start-off material was Planilux.RTM. glass (a sheet of
clear soda-lime-silica glass manufactured by Saint Gobain). The
disks (produced by grinding wheel with diamond cutter) have a
diameter of 33 mm and thickness of 2 and 4 mm. The chemical
composition of the glass is shown in Table 1, and elastic constants
and glass transition temperature [6] in Table 2. Table 2 below
reports the elastic constants and transition temperature T.sub.g of
the glass. The Young Module was measured by acoustic resonance, and
transition temperature by DSC test (according to ASTM E1346-91
norm).
2 TABLE 2 Planilux .RTM. glass properties Young Module, E (GPa)
71.4 .+-. 1.0 Poisson ratio, .nu. 0.22 Glass transition
temperature, T.sub.g (.degree. C.) 565
[0062] Given the special configuration used for the biaxial flexure
test of the disks (the main characteristic of ASTM F394-78 is the
insensitivity to edge flaws), it is not necessary to avoid edge
flaws.
[0063] The disks are annealed for 10 hours, in static air, at
510.degree. C., to remove any residual stress introduced during
manufacture. Annealing was performed by heat gradient of
245.degree. C./h and cooling by a gradient of about 40.degree.
C./h.
[0064] The ion-exchange process takes place in a semi-automatic
furnace (model TC 20 A, Lema, Parma--Italy), used simultaneously
for about 30 samples. The samples are placed in a stainless steel
basket over a bath of fused salt of about 5 litres. The kiln is
fitted with a device to keep the basket in position over the bath
for a period of pre-heating of 25 minutes, before being plunged
into the bath. After the ion-exchange process the basket is lifted
out of the bath and cooled inside the kiln for a period of 25
minutes.
[0065] The annealed disks were treated initially with pure
KNO.sub.3 at 450.degree. C. for 48 hours. This process generates a
residual-stress profile with maximum surface compression.
[0066] Subsequently the samples undergo a partial relaxation of
stress by ion-exchange in a bath of 70% mol KNO.sub.3 and 30% mol
NaNO.sub.3, at 400.degree. C. for 30 minutes.
[0067] The second treatment, as it will be described below, gives
rise to a residual-stress with the maximum compressive stress at
some distance from the surface.
[0068] The different labels depend on different thicknesses and
treatments of the samples:
[0069] Samples Y2d_tq: disks with nominal thickness 2 mm, annealed
at 510.degree. C. for 10 h
[0070] Samples Y2d.sub.--1t: disks with nominal thickness 2 mm,
annealed at 510.degree. C. for 10 h and subjected to single
ion-exchange in KNO.sub.3 at 450.degree. C. for 48 h
[0071] Samples Y2d.sub.--2t: disks with nominal thickness 2 mm,
annealed at 510.degree. C. for 10 h and subjected to ESP treatment
(ion-exchange in KNO.sub.3 at 450.degree. C. for 48 h+second
ion-exchange in KNO.sub.3/NaNO.sub.3 70/30% mol at 400.degree. C.
for 30 minutes)
[0072] Samples Y4d_tq: disks with nominal thickness 4 mm, annealed
a 510.degree. C. per 10 h
[0073] Samples Y4d.sub.--1t: disks with nominal thickness 2 mm,
annealed a 510.degree. C. per 10 h and subjected to single
ion-exchange in KNO.sub.3 at 450.degree. C. for 48 h
[0074] Samples Y4d.sub.--2t: disks with nominal thickness 4 mm,
annealed at 510.degree. C. for 10 h and subjected to ESP treatment
(ion-exchange in KNO.sub.3 at 450.degree. C. for 48 h+second
ion-exchange in KNO.sub.3/NaNO.sub.3 70/30% mol at 400.degree. C.
for 30 minutes).
[0075] Table 3 clearly illustrates the correlation between the disk
rupture disk label and its manufacturing conditions.
3 TABLE 3 Samples Annealed Single ion-exchange ESP treatment Y2d tq
X Y2d 1t X X Y2d 2t X X X Y4d tq X Y4d 1t X X Y4d 2t X X X
[0076] The following tests were carried out on the ESP disk samples
to establish their mechanical properties:
[0077] Determination of residual-stress profile and apparent
fracture toughness induced by the ESP process.
[0078] Determination of mechanical strength (or module of rupture,
MOR); biaxial flexure tests in an inert environment.
[0079] Determination of sensitivity to fatigue, n, and critical
velocity, .nu..sub.0, in distilled water.
[0080] These mechanical features are then used to design the
rupture disks.
[0081] The disks are tested for mechanical strength determination
by the "piston on 3 balls" configuration (biaxial flexure),
according to ASTM F394-78 norm, in silicone oil (FIG. 9).
[0082] The module of rupture, MOR, derived from biaxial
flexure.sup.[7] is expressed by the equation 1. 1 MOR = f = 3 P max
( X - Y ) 4 t 2 = 0.2387 P max ( X - Y ) t 2 Equation 1
[0083] The expressions of X and Y are as follows: 2 X = ( 1 + v )
ln ( B R ) 2 + [ ( 1 - v 2 ) ] ( B R ) 2 Y = ( 1 + v ) [ 1 + ln ( t
2 R ) 2 ] + ( 1 - v ) ( t 2 R ) 2
[0084] where P.sub.max (in N) is the rupture load, d (in mm) the
diameter of the support (for the 3 balls), B (in mm) the piston
radius, R (in mm) the radius of the glass disk and t (in mm) its
thickness.
[0085] To reduce the negative effect of any misalignment of the
piston with respect to the disk surface, a rubber pad of thickness
about 1 mm is placed between the piston and disk.
[0086] For dynamic fatigue tests we exploited the configuration
used for mechanical strength tests in an inert environment. The
test was carried out in distilled water, using different load
speed.
[0087] The crack velocity law's exponent (n) is determined by the
slope, m, of the line interpolating the experimental points of the
relation log .sigma..sub.f vs. log(dc/dt).sup.[8]:
n=1/m-1. Equation 2
[0088] The parameter .nu..sub.0 is calculated from the value, A
(corresponding to the intercept point on the abscissas for a value
of stress rate equal to zero), of the line interpolating the
experimental points: 3 v 0 = 2 c 0 ( n + 1 ) 0 n 10 A ( n + 1 ) ( n
- 2 ) Equation 3
[0089] where c.sub.0 is the average initial dimension of the
surface flaws.
[0090] By using a typical statistical procedure it is also possible
to calculate the error for the two parameters n and .nu..sub.0.
[0091] Mechanical Characterization of ESP Glass Disks
[0092] Residual-Stress Profile and Apparent Fracture Toughness
[0093] The residual-stress profile and apparent fracture toughness
of glass disks subjected to ion-exchange (pure KNO.sub.3 for 48
hours at 450.degree. C., Y4d.sub.--1t) and ESP disks (double
ion-exchange: 48 hours in KNO.sub.3 at 450.degree. C. and 30
minutes in a bath of 30/70% mol NaNO.sub.3/KNO.sub.3 at 400.degree.
C., Y2d.sub.--2t; double ion-exchange: 48 hours in KNO.sub.3 at
450.degree. C. and 30 minutes in a bath of 30/70% mol
NaNO.sub.3/KNO.sub.3 at 400.degree. C., Y4d.sub.--2t) are
calculated from the average of 2-3 profile measurements for each
set of samples.
[0094] Despite the maximum compression below the surface, single
ion-exchange, i.e. Y4d_lt, is not able to promote stable growth of
surface flaws. Conversely, ESP treatment gives rise to stable
growth of surface flaws. Table 4 below summarises the maximum
values of the residual-stress profile, the range of stable growth
and the predicted mechanical strength and apparent fracture
toughness curves determined on the basis of ESP theory for single
ion-exchange glass disks and ESP glass disks.
4TABLE 4 theoretical range theoretical of stable growth rupture
value sample max. compression (.mu.m) (MPa) Y4d 1t 420 MPa at 9
.mu.m absent ** Y2d 2t 569 MPa at 8 .mu.m 0.4-10 505 Y4d 2t 526 MPa
at 9 .mu.m 0.4-11 469
[0095] Mechanical Strength
[0096] As stated above, mechanical strength values were obtained
from ASTM F394-78 tests in silicone oil.
[0097] The module of rupture (MOR, .sigma..sub.f) was calculated
for each set of samples (average mechanical strength of individual
samples), with standard deviation (st; dev.) and COV, coefficient
of variation, expressed by equation 4:
COV=st.dev./MOR Equation 4
[0098] Table 5 shows the MOR for disks (tested by "piston on three
balls" configuration) obtained by using an actuator speed of 5
mm/min in an inert environment.
5 TABLE 5 sample MOR (MPa) .+-.st. dev. (MPa) COV % Y2d tq 196 49
25.0 Y4d tq 192 53 27.0 Y2d 2t 319 10 3.3 Y4d 2t 266 10 3.8
[0099] The low COV found in ESP samples is due to the stable growth
of surface flaws; this was observed with loads of up to 80% of the
rupture load. When placed in traction, the ESP glass surface showed
microscopic cracks ("microcracking effect"), confirming the stable
growth of flaws.
[0100] Fatigue Behaviour
[0101] Sensitivity to fatigue, n, and the critical velocity,
.nu..sub.0, values are calculated using the previous test
configuration in distilled water.
[0102] As stated above, the two parameters determine the influence
of sub-critical growth in the linear area governed by the power
law. High values of n and low values of .nu..sub.0 identify a
material not affected by fatigue phenomenon.
[0103] Table 6 shows sub-critical stress data for examined glasses.
ESP samples are insensitive to sub-critical crack growth.
6 TABLE 6 sample n .nu..sub.0 (.mu.m/s) Y4d tq 25.5 .+-. 2.7 780
.+-. 460 Y2d 2t 42.2 .+-. 4.5 0.37 .+-. 0.016 Y4d 2t 47.1 .+-. 11.5
4.1 .+-. 0.97
[0104] The MOR for of 4 mm ESP disks is lower than for 2 mm ESP
disks, despite similar residual-stress profiles. This is not a
surprisingly result, given that ASTM F394-78 regulations specify
thickness restrictions for samples used on the basis of Equation
1.
[0105] The conditions.sup.[9,10] for the validity of the Kirstein
and Wolley method are:
[0106] maximum deflection of the disk less than one half the
thickness;
[0107] the radius of the circumference containing the balls must be
over four times the thickness.
[0108] This means that the thickness of tested disks is between 0.2
and 3 mm; in the case of Y4d disks the requirement is not met.
[0109] To overcome this problem, MOR is calculated using the "ball
on ring" configuration. Such configuration, unlike "Piston on three
balls", uses a ring and ball of known radius, instead of a piston
(FIG. 10).
[0110] This configuration is valid for equation 1 with thickness of
over 4 mm. The only drawback is the estimate of the load area,
which--according to Hertz theory--is about 1/3 of the
thickness.sup.[9].
[0111] Table 7 shows MOR values determined in air for disks tested
by "ball on ring", at an actuator speed of 5 mm/min.
7 TABLE 7 sample MOR (MPa) .+-.st. dev. (MPa) COV (%) Y2d 1t 597 65
11 Y4d 1t 565 77 14 Y2d 2t 320 11 3.4 Y4d 2t 430 16 3.6
[0112] For the purpose of the investigation the "ball on ring"
configuration is more suitable than "piston on three balls" for two
basic reasons:
[0113] the Kirstein and Wolley formula for maximum stress remains
valid for all thicknesses (as can be seen below in relation to FEM
simulations),
[0114] the distribution of loads and restrictions is a proper
reflection of the rupture disk under pressure, as investigated by
this study.
[0115] The reasons for choosing ESP rupture disks rather than
annealed glass or glass with single ion-exchange are as
follows:
[0116] compared to the other two types of glass, ESP glass has a
low COV: this means a highly reliable rupture pressure,
[0117] the high value of n means that the glass is virtually
insensitive to fatigue phenomenon,
[0118] the particularly fine fragmentation of ESP glass, after
rupture, means that the glass can be used where this feature is a
definite requirement (as it is the case with rupture disks).
[0119] Design Criteria for the Manufacture of ESP Glass Rupture
Disks
[0120] The aim of this section is to establish design criteria for
the manufacture of rupture disks made by ESP glass.
[0121] From the analytical point of view, a circular plate subject
to uniform surface pressure will be considered.
[0122] Leaving aside the design criteria for the moment, let us
first look at one of the factors in play when deciding the
dimensions of the disk: mechanical strength, .sigma..sub.f
(MOR).
[0123] Conventionally, the features of ESP glass have been
established on the basis of two tests: monoaxial flexure tests (on
bars) and biaxial flexure tests (on disks).
[0124] Mechanical strength measured by monoaxial flexure (with
"4-point bending" configuration) is lower than the value observed
for biaxial flexure (with "Piston on 3 balls" configuration).
[0125] This is basically due to the critical nature of the edge
flaws in the former case; in biaxial tests this disadvantage is
by-passed by the particular configuration of the sample.
[0126] Therefore the problem arises as to which .sigma..sub.f must
be used. The configuration of the inflected plate under uniform
load is similar, in geometrical terms and for the load application,
to the biaxial configuration considered here ("Piston on 3 balls",
or, better still, "Ball on ring"); we will therefore use the
mechanical strength found in the two biaxial configurations as
.sigma..sub.f (MOR).
[0127] This is the starting point for establishing design criteria
for rupture disks.
[0128] Before going on to the strictly design phase, let us look at
how the literature deals with the problem of inflected plates under
uniform load.
[0129] The Poisson Model for Circular Inflected Plates
[0130] Consider a circular plate subjected to an axially
symmetrical load, p, uniformly distributed. It is reasonable to
refer the surface to polar coordinate .rho.,.theta. measuring the
inflection, w, on the z axis.
[0131] The problem is dealt with in the differential equation of
Lagrange: 4 C ( 4 w x 4 + 2 4 w x 2 y 2 + 4 w z 4 ) - p = 0
Equation 5
[0132] where C is a proportional constant.
[0133] There are a number of theories in this field; the most
useful here is the theory of Poisson. The correct form of equation
5 is as follows: 5 ( 4 w x 4 + 4 w x 2 y 2 + 4 w z 4 ) = p D
Equation 6
[0134] D is the flexional rigidity which, on the basis of the
hypothesis of Kirchhoff, for a plate with Young modulus, E, and
thickness t, is: 6 D = 1 12 Et 3 ( 1 - v 2 ) Equation 7
[0135] where:
[0136] t=constant thickness of the plate;
[0137] .nu.=Poisson ratio;
[0138] E=Young modulus;
[0139] p=uniform applied pressure;
[0140] Equation 5 is justified by applying the mechanical balance
of the elementary sector. For reasons of symmetry, there is no
shear load on the radial faces but only on the circumference
faces.
[0141] For the plate, the shear T and moments M.sub..rho.,
M.sub..theta. are referred to units of lengths, the rotation
balance yields: 7 - M d + ( M + M d ) ( + d ) d - 2 M sen d 2 d + T
d d = 0 Equation 8
[0142] and therefore: 8 M + M - M + T = 0 Equation 9
[0143] Before going on, the moments should be linked to the
displacements w. For symmetry, the displacement is irrespective of
the variable .theta., thus w=w(.rho.).
[0144] And hence: 9 = w Equation 10
[0145] and the radius, r.sub..rho., of curvature in the .rho.z
plane: 10 1 r = - 2 w 2 Equation 11
[0146] The curvature which is manifested perpendicularly to the
plane .rho.z is linked to the radius r.sub..theta. by equation 12.
11 r Equation 12
[0147] The curvatures 1/r.sub..rho., 1/r.sub..theta. are linked to
the radial linear and circumferential deformations
.epsilon..sub..rho..rho., .epsilon..sub..theta..theta. by: 12 = z r
= z r Equations 13
[0148] with corresponding tensions: 13 = Ez 1 - 2 ( 1 r + 1 r ) =
Ez 1 - 2 ( 1 r + 1 r ) Equations 14
[0149] and flexure moments M.sub..rho. and M.sub..theta. per unit
length, referring to cross-sections of the circumference and the
radius, respectively: 14 M = - t / 2 t / 2 z z = D ( 1 r + 1 r ) M
= - t / 2 t / 2 z z = D ( 1 r + 1 r ) Equations 15
[0150] Hence, on the basis of equations 13 and 14, equations 15 can
be rewritten as: 15 M = - D ( 2 w 2 + w ) M = - D ( 2 w 2 + 1 w )
Equations 16
[0151] and by replacing equation 16 in 9, the following is
obtained: 16 3 w 3 + 1 2 w 2 - 1 2 w = - T D Equation 17
[0152] or, in compact form: 17 [ 1 ( w ) ] = - T D Equation 18
[0153] If, as in our case, the pressure p is constant: 18 T = p 2
Equation 19
[0154] by direct integration of equation 18 we obtain: 19 1 ( w ) =
p 2 4 D + c 1 Equation 20
[0155] and: 20 w = p 3 16 D + c 1 2 + c 2 1 w ( ) = p 4 64 D + c 1
2 4 + c 2 ln 2 d + c 0 Equations 21
[0156] where d is the diameter of the plate support.
[0157] These equations also describe the profile conditions. Since
.rho.=0 and dw/d.rho.=0 (conditions I), then c.sub.2=0. The other
constants can be found by considering the way the plate is secured
or supported.
[0158] If the plate is fixed at the edge, conditions I, then
c.sub.1=-pd.sup.2/32D and c.sub.0=pd.sup.4/1024D, and therefore: 21
w ( ) = 1 64 p D ( d 2 4 - 2 ) 2 Equation 22 M ( ) = p 16 [ d 2 4 (
1 + v ) - 2 ( 3 + v ) ] Equation 23
[0159] In this case the maximum tension occurs on the border, i.e.:
22 ( = d 2 ) = 6 M ( = d / 2 ) t 2 = 3 16 pd 2 t 2 Equation 24
[0160] If, on the other hand, the plate is supported at the edge,
conditions II apply: 23 w = 0 2 w 2 + v w = 0
[0161] and equation 21 b becomes: 24 w ( ) = 1 64 p D ( d 2 4 - 2 )
[ 5 + v 1 + v d 2 4 - 2 ] Equation 25 M ( ) = 1 16 p ( 3 + v ) ( d
2 4 - 2 ) Equation 26
[0162] In this case the maximum tension occurs at the centre, i.e.:
25 ( = 0 ) = ( = 0 ) = 6 M ( = 0 ) t 2 = 3 32 ( 3 + v ) pd 2 t 2
Equation 27
[0163] Support/Fixing of the Plate
[0164] On the basis of the theory developed by Poisson, it is
assumed that a rupture disk is similar to a circular plate fixed at
the edges (by gasket O-rings) and is subjected to a uniform load.
The problem is now to consider whether the disk in the gasket
flange should be considered secured, supported or held weakly.
[0165] Glass is brittle thus we can assume the plate breaks under
pressure p when one of .sigma..sub..rho..rho.,
.sigma..sub..theta..theta. is equal to .sigma..sub.f as determined
by biaxial flexure tests.
[0166] If the disk is secured (FIG. 11), the value of
.sigma..sub..rho..rho. is larger close to the edge, i.e.: 26 max =
k 1 ( d t ) 2 p Equation 28
[0167] where:
[0168] d=disk support diameter;
[0169] t=disk thickness;
[0170] k.sub.1=stress factor k.sub.1=0.1875 close to the edge.
[0171] One must remember that d is the diameter of the disk
support; the disk has a diameter of 2R, just a little larger than
d.
[0172] If the disk is supported (FIG. 12), the value of
.sigma..sub..rho..rho., .sigma..sub..theta..theta. is larger at the
centre, i.e.: 27 max = max = k 2 ( d t ) 2 p Equation 29
[0173] where:
[0174] d=disk support diameter;
[0175] t=disk thickness;
[0176] k.sub.2=stress factor k.sub.2=0.3025 at the disk centre.
[0177] As it will be shown below, there is a distribution
parallelism between the forces acting on a disk under pressure and
a disk loaded at the centre with a concentrated force.
[0178] Numerous preliminary rupture tests were carried out on
annealed and ESP glass, using the system shown in FIG. 13,
wherein:
[0179] Support flange (3,7) tightened by nuts,
[0180] Rupture disk (5),
[0181] O-ring (4,6),
[0182] Rubber disk for the placement of the ball (2),
[0183] Stainless steel ball (1).
[0184] By analysing the fracture mode and rupture loads, the disk
assembled on two flanges with O-rings can be considered to be
supported at the edges.
[0185] The use of flanges (with O-rings) is equivalent to
supporting the disk, so the formula for design purposes is as set
out below.
[0186] Design Criteria
[0187] It is likely that disk failure occurs when the maximum
radial and circumferential tensile stress (which are equal at the
centre of the plate) reach the rupture value .sigma..sub.f,biax
(derived from biaxial configurations).
.sigma..sub..rho..rho..sup.max=.sigma..sub.f,biax=.sigma..sub..theta..thet-
a..sup.max Equation 30
[0188] The rupture pressure of the disk (p.sub.max) is obtained by
replacing 30 in 29: 28 p max = f , biax k 2 ( t d ) 2 Equation
31
[0189] The ESP glass disk may be subjected to constant operating
pressure (p.sub.op) as shown.sup.[4] in FIG. 14 (tempered glass).
It breaks--with a fine fragmentation--for an increase of pressure
to .DELTA.p=p.sub.max-p.sub.op.
[0190] There are two main parameters to be considered:
[0191] mechanical strength, .sigma..sub.f,biax
[0192] geometrical ratio, t/d.
[0193] There are two possible assembly configurations for rupture
disks depending on whether the safety device is designed for high
or low pressure systems.
[0194] In both cases the disk size is calculated by using equation
31 with k.sub.2=0.3025: 29 p max = 3.30 f . biax ( t d ) 2 Equation
32
[0195] In order to properly apply the proposed design criteria it
is necessary not to confuse the disk support diameter (the O-ring
in this case), d, with the glass disk diameter, 2R.
[0196] FIG. 14 should then be used to make sure the t/d ratio,
derived for p.sub.max, is sufficient for safety at pop operating
regimens.
EXAMPLE
[0197] Suppose that we have a conduit carrying steam at a pressure
of 2 atm (0.2 MPa) and we need to design an ESP glass rupture disk
(let us say with .sigma..sub.f,biax=350.+-.15 MPa) with p.sub.max
of 25 atm (2.5 MPa) starting from a plate with nominal thickness of
2 mm (t=1.58 mm).
[0198] We can now draw the graph of equation 32 on the basis of
p.sub.max and d. With p.sub.max of 2.5 MPa d=34 mm. Considering the
standard deviation (15 MPa) of .sigma..sub.f,biax the diameter can
be calculated to an accuracy of +1 mm, equivalent to a pressure
accuracy of .+-.0.15 MPa.
[0199] It should then be verified that the operating pressure of
the system is less than or equal to the limit pressure (shown in
FIG. 14).
[0200] From FIG. 14, related to tempered glass and a t/d ratio of
0.046, the limit pressure is about 0.7 MPa: above the operating
pressure of 0.2 MPa for the system.
[0201] This further protects the system against other cases of
failure. For a correct manufacture of this device a glass disk
diameter, 2R, just slightly greater than the O-ring (support)
diameter, d, should be used.
[0202] Simulation by Finite Elements Method (FEM)
[0203] To check the design criteria, the rupture disks should be
tested on a high pressure test bench.sup.[1]. Due to the high
pressures involved (up to 300 MPa) a simulation is used with test
conditions that are different from the operating conditions of the
disk supported at the edges but with similar stress field
distribution.
[0204] Let us look at the following situations:
[0205] Disk under pressure, with supported edge:
[0206] As stated before: 30 ( = 0 ) = ( = 0 ) = 3 32 ( 3 + v ) pd 2
t 2 Equation 33
[0207] the maximum tensile stress occurs close to the centre of the
disk.
[0208] Disk with concentrated load, Q, at the centre, with
supported edge:
[0209] This configuration is difficult to be achieved due to the
difficulty of applying the load at the precise central point.
Simpler is the case of a load applied by means of a steel ball.
[0210] This situation is comparable to the "Ball on ring test" and
therefore the maximum tensile stress at the centre is calculated by
Kirstein and Wolley.sup.[7] as follows: 31 max ( = 0 ) = 3 Q ( 1 +
v ) 4 t 2 [ 1 + 2 ln d 2 b + 1 - v 1 + v ( 1 - 2 b 2 d 2 ) d 2 4 R
2 ] Equation 34
[0211] where:
[0212] Q=applied load
[0213] t=thickness of the plate
[0214] d=ring diameter
[0215] b=radius of the ball/disk contact area
[0216] R=disk radius.
[0217] To estimate the contact area, b, Kirstein and Wolley use a
b* value (instead of b in equation 34) equal to t/3.
[0218] On the basis of the obtained results, the tension at the
centre of the disks in the two situations is the same if, and only
if: 32 3 Q ( 1 + v ) 4 t 2 [ 1 + 2 ln d 2 b + 1 - v 1 + v ( 1 - 2 b
2 d 2 ) d 2 4 R 2 ] = 3 32 ( 3 + v ) pd 2 t 2 Equation 35
[0219] giving the ratio between applied pressure, p, and load, Q,
simulating the maximum stress and hence the rupture of the disk. 33
p = 8 ( 1 + v ) ( 3 + v ) d 2 [ 1 + 2 ln d 2 b + 1 - v 1 + v ( 1 -
2 b 2 d 2 ) d 2 4 R 2 ] Q = K ( d , R , t ) Q Equation 36
[0220] where K is a parameter depending on d, R and t.
[0221] This similarity, expressed by equation 36, is confirmed a
FEM (Finite Element Method) simulation performed by Ansys.COPYRGT.
program.
[0222] The first case includes comparison between the results
according to equation 36 and the simulation, for a disk with the
following features:
[0223] E=71.4 GPa, .nu.=0.22
[0224] 2R=30 mm, d=27 mm, t=1.5 mm, b*=t/3=0.5 mm.
[0225] FEM simulation shows:
p=0.0101Q Equation 37
[0226] and equation 36:
p=0.0107Q Equation 38
[0227] with Q in N and p in MPa.
[0228] The second case includes the comparison between the results
obtained according to equation 36 and the simulation, for a disk
with the following features:
[0229] E=71.4 GPa, .nu.=0.22
[0230] 2R=30 mm, d=27 mm, t=3.8 mm, b*=t/3=1.27 mm.
[0231] FEM simulation shows:
p=0.0083Q Equation 39
[0232] and equation 36:
p=0.0083Q Equation 40
[0233] with Q in N and p in MPa.
[0234] These results show that it is possible to use the
configuration of FIG. 15 to find the load Q which, in terms of
rupture, simulates the pressure p of the configuration of FIG. 16
(rupture disk under pressure between two flanges with O-rings).
[0235] Experimental Verification
[0236] To check the accuracy of the design criteria, rupture disks
were tested as shown in FIG. 15 and subsequently by high pressure
experimental bench tests carried out for that purpose.
[0237] The schematic of the bench tests is shown in FIG. 17.
[0238] The dimensions of the components used on the test bench were
as follows:
[0239] Diameter of inner flange d=27 mm
[0240] Diameter of rupture disk 2R with thickness t
[0241] Diameter of stainless steel ball=6 mm.
[0242] The concentrated load Q is applied by a mechanical testing
machine with controlled displacement. After establishing the
mechanical features on the basis of geometry, it is possible to
calculate the loads Q simulating a given rupture pressure on the
basis of the size of the rupture disk. ESP glass disks were tested
with diameter 2R of 33 mm and thicknesses of 2 and 4 mm.
[0243] Table 8 shows the geometrical data for the disk, the
predicted Q.sub.max according to equation 34 (with MOR for the two
biaxial tests), the measured Q.sub.max according to the
configuration of FIG. 15 and the p.sub.max simulated by the load
Q.sub.max (according to equation 36). Specifically, Table 8 reports
the results of experimental verification--R and t being the radius
and thickness of the ESP disk, respectively, Q.sub.max.sup.P3B the
predicted rupture load on the basis of the MOR established by the
"Piston on three balls" test, Q.sub.max.sup.BOR the predicted
rupture load on the basis of the MOR established by "Ball on ring"
test, p.sub.max.sup.predicted the predicted rupture pressure on the
basis of equation 36 and p.sub.max.sup.measured the measured
rupture pressure on the basis of equation 32 (d=27 mm, see the
configuration of the FIG. 15).
8TABLE 8 Q.sub.max.sup.BOR Q.sub.max.sup.meas.
p.sub.max.sup.predicted p.sub.max.sup.measured R (mm) t (mm)
Q.sub.max.sup.P3B (N) (N) (N) (MPa) (MPa) 16.5 1.60 357 358 357 3.7
3.7 16.5 1.56 336 338 326 3.5 3.4 16.5 1.61 362 363 374 3.8 3.9
16.5 1.60 355 356 349 3.7 3.6 16.5 1.61 360 361 333 3.8 3.5 16.5
1.63 370 372 373 3.9 3.9 16.5 1.61 358 359 344 3.7 3.6 16.5 1.56
335 337 351 3.5 3.7 16.5 3.84 2195 3546 3467 28.8 28.2 16.5 3.84
2193 3541 3307 28.7 26.9 16.5 3.85 2199 3552 3670 28.8 29.8 16.5
3.82 2171 3507 3521 28.5 28.7 16.5 3.85 2201 3554 3505 28.8 28.5
16.5 3.84 2198 3550 3608 28.8 29.3 16.5 3.85 2203 3558 3563 28.8
28.9 16.5 3.84 2198 3550 3516 28.8 28.6 16.5 3.84 2187 3533 3504
28.8 28.5 16.5 3.84 2198 3550 3507 28.7 28.5 16.5 3.84 2194 3543
3577 28.8 29.1 16.5 3.84 2193 3541 3535 28.5 28.7 16.5 3.85 2203
3558 3503 28.8 28.5 16.5 3.84 2195 3546 3496 28.8 28.4 16.5 3.85
2199 3552 3554 28.8 28.9
[0244] It is clear from the experimental data that the MOR measured
by the "Piston on three balls" test cannot be used at the design
stage. On the other hand, using the MOR derived from the "Ball on
ring" test, the values for Q.sub.max are in line with experimental
data.
[0245] Experimental Check on High Pressure Test Bench
[0246] The last check carried out on the disks is an operating test
on a high pressure test bench created especially for that
purpose.
[0247] The equipment comprises:
[0248] a manual water pump (capacity 1 litre) able to reach a
maximum pressure of 385 bar
[0249] an instantaneous pressure read-out (analogical gauge from
0-400 bar)
[0250] a flange containing a disk under pressure.
[0251] The flange assembly was designed specifically for an
operating configuration with a pure support (by inner O-ring) for
the glass disk.
[0252] Rupture tests were carried out (at high load speeds) on
annealed, single ion-exchanged and ESP glass disks.
[0253] Table 9 shows the predicted and measured rupture pressure
values for ESP disks. Specifically, Table 9 reports the rupture
pressure values derived from high pressure experimental bench tests
compared with the values predicted by design criteria.
9 TABLE 9 disk ESP p.sub.max.sup.predicted (bar)
p.sub.max.sup.measured (bar) Y2d 2t 38 35-40 Y4d 2t 252 240-260
[0254] These results give rise to useful considerations about the
fragmentation of the glass on failure of the rupture disk.
[0255] Upon failure ESP glass rupture disks are more finely
fragmented (in the maximum load area) than annealed disks. Disks
which have undergone single ion-exchange also have fairly fine
fragmentation (due solely to the high elastic energy in the
breaking phase) but do not match ESP glass disks (whose
fragmentation is due more to microcracking than to stored elastic
energy).
[0256] Of course, without prejudice to the underlying principle of
the invention, the details and embodiments may vary, also
significantly, with respect to what has been described and shown,
by way of examples only, without departing from the scope of the
invention, as depicted by the claims that follow.
REFERENCES
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Requisiti generali", September 1993.
[0258] [2] ASME Code, section VIII, div. I, edition 1998.
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McGraw Hill, 3rd ed., 1984.
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[0268] [12] V. M. Sglavo, D. Green, "Flaw-Insensitive
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[0269] [13] V. M. Sglavo, D. Green, "Flaw-Insensitive
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[0270] [14] A. Y. Sane, A. R. Cooper, "Stress Builup and Relaxion
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[0271] [15] C. W. Sinton, W. C. LaCourse, M. J. O'Connell,
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* * * * *